"Hellosailor posits that"
No, he does not. In fact he never knowingly "posits" anything. He's merely stated that there are repeated estimates provided by other sources that give that number. That's not "posit"ing.

Now, johnny, let me also explain to you that your math is trash. You're comparing the surface area of shipping containers to that of the whole ocean. Totally invalid comparison, since the shipping containers we are concerend with are lost "in the shipping lanes" not randomly dispersed about the oceans. If you want to compare surface areas in order to estimate the chance of a collision, FIRST you need to examine the surface area of the shipping lanes where the containers are "launched". Then you need to extrapolate forward with current and drift, to determine the surface area of the places where they will go. Next you need to determine where recreational boaters go, because in some places they are basically IN those same shipping lanes, while in other places (like the North Sea) recreational vessels are damn rare.

If you do not adjust your "surface area" for dispersal and use patterns, but simply gob it all up, then the math is junk. Your answer will be off by many scales of magnitude.

What are my odds of digging up a corpse when I dig in the sand at the beach? Oh, well, we start with the size of a coffin, and ignore the fact that most of those are in designated graveyards, not just randomly scattered about?